(1) Field of the Invention
The present invention relates to a full-contact type image sensor and a method for fabricating the same, and more particularly to a full-contact type image sensor which is mounted on an image input unit, such as a facsimile set or a hand scanner, and which is operable to read an original document in essentially full contact therewith.
(2) Description of the Related Art
Various image sensors for reading image data of an original document have been well known in the art, and full-contact type image sensors which are small in size, are used instead of image sensors using charge coupled devices (CCDs) particularly for hand scanners and facsimile image sensors which are required to be small in size.
Applicant's Corporation has proposed a full-contact type image sensor in which an image sensor with a large number of photoelectric conversion elements formed on a transparent substrate and having apertures is combined with an optical fiber converging member (Japanese Patent Application Kokai Publication No. Hei 6-291935). FIG. 1 shows such an example of the prior art full-contact type image sensor proposed by the Applicant's Corporation. The full-contact type image sensor as shown essentially comprises a light source 1, a number of photoelectric conversion elements 3 provided in an array on a transparent substrate 2, and an optical fiber converging member 8 including a number of optical fibers 9 bundled in an array.
The photoelectric conversion element 3 includes an upper opaque common electrode common to all pixels, lower transparent individual electrodes corresponding to the respective pixels, a photosensitive layer interposed between the upper and lower electrodes, and a number of apertures 5 penetrating a photoelectric conversion region 4 constituted by the electrodes and photosensitive layer.
Next, the operation of the full-contact type image sensor described above will be explained. Light emitted from the light source 1 is transmitted through the transparent insulating substrate 2 and the apertures 5 formed in the photoelectric conversion elements 3 and then through a transparent adhesive layer 7. After passing through the optical fibers 9, the light is incident on an original document 10. The incident light whose incidence angles on the optical fibers 9 are smaller than the aperture angle determined by the numerical aperture (NA) of the optical fibers 9, is repeatedly totally reflected inside the optical fibers 9, thus illuminating the original document 10. Light incident at angles greater than the aperture angle mentioned above, is transmitted through and scattered by the walls of the optical fibers 9, thus causing stray light which deteriorates the image quality.
Of the light reflected from the original document 10 and incident on the optical fibers 9, those components with incidence angles smaller than the aperture angle pass through the optical fibers 9 and are incident on the photoelectric conversion region 4 (photosensitive layer) for photoelectric conversion. With this conventional full-contact type image sensor, since the optical fibers 9 are used for both the illumination of the original document 10 and the transmission of the reflected light therefrom, high resolution image input can be realized.
FIG. 2 is a graph showing the results of calculated resolution for the full-contact type image sensor having the structure as shown in FIG. 1. In the graph, the ordinate is the resolution (Modulation Transfer Function--MTF), and the abscissa is the distance (mm) between the optical fiber converging member 8 and the original document 10. NA is the numerical aperture of the optical fibers 9. As can be seen from FIG. 2, for the same numerical aperture, the resolution deteriorates if the distance between the optical fiber converging member 8 and the original document 10 is increased. That is, the further the optical fiber converging member 8 is separated from the original document 10, the lower the resolution becomes. As can be seen from FIG. 2, this means that the resolution deterioration can be reduced if the optical fiber 9 having a smaller NA is used.
FIG. 3 is a graph with the ordinate representing signal amount of the full-contact type image sensor having the structure as shown in FIG. 1 and the abscissa representing the NA of the optical fiber 9. As is seen from FIG. 3, with large values of NA, the original document 10 can be illuminated with a larger amount of light. In addition, the light reflected from the original document 10 can be led more to the photoelectric conversion region 4 and the signal amount is increased. As is seen from FIGS. 2 and 3, in the full-contact type image sensor having the structure as shown in FIG. 1, the resolution and the signal amount are traded-off with each other.
With an aim of suppressing the resolution deterioration resulting from the separation of the optical fiber converging member from the original document, a full-contact type image sensor has been proposed, which uses a micro-lens array plate provided between an optical fiber converging member and an original document (Japanese Patent Application Kokai Publication No. Hei 3-265356). FIG. 4 is a sectional view showing an example of the prior art contact type image sensor having this structure. As shown, this prior art contact type image sensor comprises a light source 71, a photoelectric conversion element array 72, an optical fiber converging member 73, a micro-lens array plate 75 disposed thereunder, and a light-blocking layer 76 interposed between the optical fiber converging member 73 and the micro-lens array plate 75.
Next, the operation of the conventional full-contact type image sensor described above will be explained. Light emitted from the light source 71 is transmitted obliquely through the optical fiber converging member 73 and the micro-lens array plate 75 to illuminate the original document 77. Light dispersedly reflected from the original document 77 is re-directed by the micro-lens array plate 75 so that the angle with respect to the axial direction of the optical fibers 74 is reduced. Then the light is incident on the optical fiber converging member 73 and thence is incident on the photoelectric conversion element array 72 for photoelectric conversion.
With the prior art full-contact type image sensor as shown in FIG. 4, it is possible to suppress the resolution deterioration as a result of the refracting action of the micro-lens array plate 75 on the reflected light from the original document 77.
With the conventional full-contact type image sensor as shown in FIG. 1, high resolution is obtainable because the optical fibers 9 are used for both the illumination of the original document 10 and the detection of the reflected light from the original document 10. On the demerit side, however, the resolution as determined by the NA of the optical fiber 9 and the signal amount are traded-off with each other as described above before in connection with FIGS. 2 and 3. Therefore, it is very difficult to optimize both the resolution and the signal amount.
Also, it is desired, with the full-contact type image sensor as shown in FIG. 1, to provide a large aperture angle of the optical fibers 9 in order to provide increased light utilization efficiency and suppress stray light. However, with the large aperture angle optical fibers, the light reflected from areas other than those in the original document where the reading-out is not interested, also reaches the photoelectric conversion region 4 after passing through the space 11 between the original document 10 and the optical fiber converging member 8, thus resulting in deterioration of the resolution. No problem arises when the space 11 is not present. This, however, requires the original document 10 to fully contact the optical fiber converging member 8. Particularly, in an application for a hand scanner, the resolution is deteriorated even with a slight separation of the optical fiber converging member 8 from the original document 10, thus making it inconvenient to use.
In the prior art full-contact type image sensor shown in FIG. 4, light emitted from the light source 71 is transmitted obliquely through the optical fiber converging member 73 and the micro-lens array plate 75 to illuminate the original document 77. Therefore, a large amount of stray light is generated as a result of scattering of light by the wall surfaces of the optical fibers 74, and high resolution cannot be obtained.
Besides, the micro-lens array plate 75 has a thickness of the order of several tens of microns for it is fabricated by such processes as mechanical processing or die molding a plastic material, glass or other such transparent materials. This means that close contact is sacrificed by a distance corresponding to the thickness of the microlens array plate 75. In other words, with the prior art full-contact type image sensor shown in FIG. 4, the close contact of the micro-lens array plate 75 with the original document 77 is equivalent to the separation of the optical fiber converging member 73 from the original document 77 in the order of several ten microns, and this means deterioration of the resolution to a corresponding extent.
In another aspect, for efficient transmission of light reflected from the original document 77, it is desirable that the micro-lenses in the micro-lens array plate 75 and the optical fibers 74 in the optical fiber converging member 73 are in one-to-one correspondence with one another. However, their mutual one to one positioning is quite difficult, and is impossible particularly in the case where the optical fibers 74 of the optical fiber converging member 73 are subject to deformation during manufacture.